For powering a LED panel, comprising an array of LEDs, it is traditionally possible to transfer electric power from a source to the LEDs via wires, but this is rather complicated and expensive. Further, for illumination purposes it is typically desirable that all LEDs have mutually the same light output, but it is complicated to achieve this in a wired embodiment. It is to be noted that the individual LED components do not necessarily have mutually identical characteristics: manufacturing tolerances will cause one LED to be brighter than the other, and this difference should be eliminated as much as possible.
In an alternative design, the LEDs are, either individually or as a group, provided with series capacitors for limiting the LED current. Tolerances in these series capacitors will cause variations in the light output between LEDs, and for compensation additional capacitors can be used. U.S. Pat. No. 7,830,095 describes a system where such LEDs are provided with a plurality of mutually parallel capacitors, each capacitor provided with a switch, so that it is possible to adapt the series capacitance value by selectively making or braking one or more of these switches. A problem is, however, that the capacitance variations, and hence the LED current and hence the LED output, can only be varied stepwise. Further, for precise compensation, many trimming capacitors with many corresponding switches are needed, which is expensive, and this problem increases with increasing spread of the LEDs and/or increasing spread of the series capacitors.
In case resonant powering is used, a supply device comprises an AC power generator for generating AC power, and at least one inductor coupled in series with respective series capacitors for the respective LEDs or groups of LEDs. It should be clear to a person skilled in the art that in such case the impedance of the LED array as a whole, and the resonance frequency of the LED array as a whole, will vary with the capacitance variations.